A spark plug is known having a built-in resistor to suppress radio noise generated by spark discharge (see, for example, Japanese Laid-Open Patent Publication No. 2015-64987). This type of spark plug includes: an insulator formed with an axial hole in which the resistor is arranged; a metal shell partially surrounding an outer circumferential surface of the insulator; a ground electrode joined to a front end of the metal shell; a center electrode inserted in the axial hole of the insulator; and a conductive seal held in contact with the center electrode and the resistor. There is a spark gap defined between a front end of the center electrode and the ground electrode so that a flame kernel is produced in the spark gap at the time of spark discharge.
The above conventional spark plug has the problem that, at the time of spark discharge, electric charge accumulated in a parasitic capacitance between the metal shell and the conductive seal or the center electrode moves to the spark gap and accelerates wear of the center electrode and the ground electrode (generically referred to as “electrode wear”).
In order to decrease the amount of the electric charge that accelerates electrode wear, it is conceivable to reduce the parasitic capacitance by decreasing the area of the conductive seal. However, this leads to a decrease in the contact area between the conductive seal and the center electrode so that the state of contact between the conductive seal and the center electrode becomes deteriorated by impact or vibration (that is, the spark plug becomes deteriorated in impact resistance).
The present invention has been made to address the above problems. An advantage of the present invention is a spark plug capable of preventing electrode wear while ensuring impact resistance.